1. Field of the Invention
The present invention relates to signal detecting devices, and more particularly, to a device employed in a communication apparatus comprising an automatically dialling transmitting function the signal detecting device detecting a identification signal which is a composite wave of a plurality of frequencies transmitted from an exchange system via a telephone line.
2. Description of the Background Art
Many communication apparatus connected to a public telephone line such as a facsimile apparatus are provided with automatically dialling transmitting functions. Regarding the procedure of connection with a called device by the automatically dialling transmitting function, a communication apparatus receives an identification signal transmitted from an exchange of a public telephone line or a private branch exchange (a PBX, referred to as "exchange system" hereinafter), to send a dial signal after making determination that the exchange system is at a state that can receive a dial signal.
The above-mentioned identification signal comprises the so-called dial tone and busy tone. A composite signal which is the composition of two different constant frequency signals is generally used as an identification signal.
A conventional signal detecting device for detecting this composite signal is shown in FIG. 7.
The signal detecting device of FIG. 7 is implemented to detect a composite signal that is a composition of a signal of frequency f.sub.1 and a signal of frequency f.sub.2. The signal detecting device comprises a CML (Connect Model to Line) relay 1, a matching transformer 2, a modem 3, an amplifier 4, tone detectors 5 and 6, and an AND gate 7. CML relay 1 is a switching circuit for selectively connecting the telephone line to either of matching transformer 2 or amplifier 4. Matching transformer 2 is inserted between CML relay 1 and modem 3 for matching the characteristic impedance of the telephone line, and for insulating modem 3 from the telephone line. Amplifier 4 amplifies the signal transmitted from the exchange system in the automatically dialling transmission. Tone detector 5 detects the signal component of frequency f.sub.1 from the output of amplifier 4. Tone detector 6 detects the signal component of frequency f.sub.2 from the output of amplifier 4. AND gate 7 performs an operation of the logical product of the outputs of tone detectors 5 and 6.
As shown in FIG. 8, tone detectors 5 and 6 provide a logic 0 signal when a frequency component within a range (f.sub.0 .+-..DELTA.f) centered about the frequency (f.sub.0) to be detected is applied, and a logic 1 signal when a frequency component outside the above mentioned range is applied.
When the outputs of tone detectors 5 and 6 are both logical 0, a composite signal comprising the frequency components of f.sub.1 and f.sub.2 from a telephone line is received by the device. At this time, AND gate 7 provides a logic 0 signal.
The output of AND gate 7 is applied as a control signal of an automatically dialling transmitting device (not shown). The automatically dialling transmitting device receives the logic 0 signal from AND gate 7 to make determination that the exchange system is at a state that can receive a dial signal. Then, a dial signal is transmitted.
The above described conventional signal detecting device had a disadvantage that the circuit structure is complicated since two tone detectors having complex structures are used. If the exchange system transmits a signal of another frequency of f.sub.3 as another identification signal in addition to the above described composite signal, another tone detector must be provided for detecting frequency f.sub.3 to result in a further complicated circuit. The structure of the tone detector is disclosed in, for example, laid-open Japanese Patent applications No.48-93203, No. 48-93204, and No.48-93205.